1. Field of the Invention
The present invention relates to a method and a control device for controlling a self-igniting internal combustion engine.
2. Description of Related Art
Self-igniting combustion methods, also known as HCCI methods (Homogenous Charge Compression Ignition) or CAI methods (Controlled Auto Ignition), are distinguished by an economical consumption of fuel, in particular in partial load situations, and by relatively low raw pollutant emissions. Thus, in a self-igniting internal combustion engine it is possible to do without an additional, relatively expensive exhaust gas aftertreatment, for example using a NOx storage catalytic converter.
In a self-igniting combustion method, the fuel injected into the internal combustion engine is mixed with hot exhaust gases and is then automatically ignited during a compression. This results in a relatively low combustion temperature, with a large number of exothermic centers in the combustion chamber, and thus to a very uniform and rapid combustion.
As a rule, self-igniting engines are equipped with direct gasoline injection. In addition, self-igniting engines have a variable valve system. A distinction is made between fully variable valve systems, for example having an electrohydraulic valve controlling, and partially variable valve systems, which can be realized by a camshaft-controlled valve operation. The latter is the more economical alternative.
In order to execute a self-igniting combustion method, a particular quantity of exhaust gas is held back in the cylinder or is recirculated back into the cylinder and is used for the initiation of the combustion during the compression phase. One speaks here of an internal or an external exhaust gas quantity. The internal exhaust gas quantity is held back in the cylinder by a negative valve overlap. In contrast, the external exhaust gas quantity can be fed back or can be suctioned back by a brief opening of the outlet valve during the intake phase.
In a self-igniting combustion method, however, the direct trigger in the form of an external ignition for the initiation of the combustion is not present. The position of the combustion, which is often called the combustion position, can therefore be influenced only by a carefully adjusted controlling of the CAI engine system. In order to determine the combustion position, a measurement value is often used that is determined via a cylinder pressure sensor. For example, this measurement value relates to a specific energy conversion point that is given as a rule by a crank angle. One often speaks here of a center of combustion MFB50 (mass fraction burned 50%).
As a rule, CAI combustion methods include a cycle-to-cycle correlation with a particular temperature, on the basis of the internal and/or external exhaust gas quantity coming from the previous cycle. For example, a premature combustion results in a slight decrease in the temperature of the internal and/or external exhaust gas quantity in the following cycle. This retards the combustion, and therefore often results in a late combustion. As a result, the temperature of the internal and/or external exhaust gas quantity in the next cycle may be too high, and may again cause a premature combustion that occurs even earlier than the previous premature combustion. Deviations of the combustion position relative to a target combustion position may continue to increase in this way until the combustion comes to a complete halt. In particular in low-load operation, close to no-load operation, the risk of a failure of combustion is relatively great.
In addition, a very late position may also be accompanied by incomplete combustion. In the subsequent intermediate compression, there is then the risk that the HC/CO molecules that did not react in the previous cycle will react exothermically with the remaining oxygen. This often results, in the next combustion, to a significantly early and loud combustion, due to the increased temperature of the internal and/or external exhaust gas quantity.
Therefore, it would be desirable to have the possibility of ensuring a reliable maintenance of a desired target combustion position during operation of a self-igniting internal combustion engine.